Influence of In-medium properties of vector mesons on dilepton production in heavy ion collisions at SIS energies

Dileptons represent a unique probe for nuclear matter under extreme conditions reached in heavy ion collisions. They allow the study of meson properties, like mass and decay width, at various density and temperature regimes. Up to now, in the Tübingen model for dilepton production, modification of meson properties in nuclear medium has been accounted for by allowing a density dependence of the mass (Brown–Rho scaling) together with an ad hoc dependence of the meson decay widths on the same variable. We use the extended vector meson dominance (eVMD) model to extract meson properties in nuclear matter by computing the in-medium meson spectral functions. Dilepton spectra for C+C at 1.0 and 2.0 AGeV are calculated and compared with previous results.     

Electromagnetic radiation from hot and dense hadronic matter

The modifications of hadronic masses and decay widths at finite temperature and baryon density are investigated using a phenomenological model of hadronic interactions in the Relativistic Hartree Approximation. We consider an exhaustive set of hadronic reactions and vector meson decays to estimate the photon emission from hot and dense hadronic matter. The reduction in the vector meson masses and decay widths is seen to cause an enhancement in the photon production. It is observed that the effect of p-decay width on photon spectra is negligible. The effects on dilepton production from pion annihilation are also indicated.     

Search for direct photons from S-Au collisions at 200 GeV/nucleon by the CERES Collaboration

In this note we comment on a recent publication in this journal by the CERES (NA45) Collaboration [1]. The authors report to have determined an upper limit on the direct photon yield relative to the decay photon yield in S + Au collisions of 14% and 7% by two different methods, respectively. We argue that these limits are unsupported by the results and analysis of the CERES data. The systematic error estimates quoted in the CERES analysis are consistently overly optimistic. Using more realistic estimates of the various error contributions and propagating them appropriately we arrive at a direct photon upper limit which at best is 20% of the inclusive photon yield, and most probably is much higher.     

Electromagnetic signals of quark gluon plasma

Successive equilibration of quark degrees of freedom and its effects on electromagnetic signals of quark gluon plasma are discussed. The effects of the variation of vector meson masses and decay widths on photon production from hot strongly interacting matter formed after Pb + Pb and S + Au collisions at CERN SPS energies are considered. It has been shown that the present photon spectra measured by WA80 and WA98 Collaborations can not distinguish between the formation of quark matter and hadronic matter in the initial state.     

Low mass dileptons from Pb + Au collisions at 158 A.GeV

The medium modification of vector meson properties in hot/dense hadronic matter and its role in explaining the CERES/NA45 dilepton data at different centralities are discussed.     

Theoretical analysis of dilepton spectra in heavy ion collisions at GSI-FAIR energies

This paper addresses the theoretical analysis of dilepton spectra in C+C collisions at GSI-SIS energies. Theoretical predictions for the recent data of the HADES Collaboration at SIS energies are made with the help of a hadron-string transport model, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. A mass shift of the ρ meson due to kinematical effects is discussed.     

Φ Meson Production in Heavy Ion Collisions at RHIC

We present Φ meson production in Cu+Cu and Au+Au collisions measured by the STAR experiment at RHIC.The hadronic decay mode Φ→K~+K~- is used in the analysis.The yields for Φ meson in Cu+Cu and Au+Au collisions at a given beam energy are scaled by the number of participant.The N_(part) normalized Φ meson yields in heavy ion collisions over those from p+p collisions are larger than 1 and increase with collision energy.These results suggest that the source of enhancement of strange hadrons is related to the formation of a dense medium in high energy heavy ion collisions and can not be only due to canonical suppression of their production in smaller systems.We also present STAR results on the Φ meson elliptic flow υ_2 from 2~(1/SNN)=200 GeV Cu+Cu at RHIC.The elliptic flow in Cu+Cu system that has the similar relative magnitude and qualitative features as that in Au+Au system.The observations imply the hot and dense matter with partonic collectivity has been formed in heavy ion collisions at RHIC.However,eccentrality normalized υ_2,υ_2/(n_qε_(part)) is lower for Cu+Cu than for Au+Au collisions at 200 GeV.So this might indicate thermalization has not been reached in 200 GeV Cu+Cu collisions.     

Open charm measurements at STAR

We report on the measurements of D⁰ meson production via direct reconstruction through the hadronic decay channel D⁰→Kπ in minimum bias d+Au and Au+Au collisions at with p_T up to ∼3 GeV/c. We derive the charm production total cross-section per nucleon–nucleon collision from a combination of three measurements: the D⁰ meson spectra, the non-photonic electron spectra from charm semi-leptonic decays obtained in p+p, d+Au, and Au+Au collisions, and the charm-decayed single muon (prompt muon) spectra at low p_T in Au+Au collisions. The cross-section is found to follow binary scaling, which is a signature of charm production exclusively at the initial impact. The implications of charm quark energy-loss and thermalization in the strongly interacting matter are discussed. PACS 25.75.Dw; 13.20.Fc; 13.25.Ft; 24.85.+p     

Sensitivity of electromagnetic spectra to equation of state and initial energy density in the Pb + Pb collisions at SPS

We study Pb + Pb collisions at 158 A GeV/c using hydrodynamical approach. We test different equations of state (EoSs) and different initial conditions and show that there are more than one initial state for each EoS which reproduce the observed hadronic spectra. We also find that different equations of state favor different freeze-out temperature. Simultaneously we calculate the thermal dilepton and photon spectra for each EoS and initial state. We compare the dilepton mass spectrum to data measured by the CERES collaboration and find that the differences in spectra obtained using different EoSs and initial states are not resolvable within the current experimental resolution. However, at invariant masses over 2 GeV the difference in the yield due to various initial states is close to an order of magnitude. We also study the rapidity distribution of lepton pairs and find that for masses around 800 MeV the shape of the distribution depends strongly on the EoS.     

Dile pton production in p+ p,Cu+Cu and Au+Au collisions at $\sqrt{s} = 200$ GeV

We study dilepton production in proton–proton, Cu+Cu as well as in Au+Au collisions at the center-of-mass energy \(\sqrt{s} =200\) GeV per participating nucleon pair within an extended statistical hadronization model. In extension to earlier studies we incorporate transport calculations for an estimate of uncorrelated e ⁺ e ^? -pairs from semileptonic D meson decays. While the invariant mass spectrum of dielectrons is well understood in the p+p collisions, severe discrepancies among different model scenarios based on hadronic degrees of freedom and recent data from the PHENIX Collaboration are found in heavy-ion collisions in the low-mass region from 0.15 to 0.6 GeV as well as in the intermediate mass regime from 1.1 to 3 GeV when employing the standard dilepton sources. We investigate, furthermore, the background from correlated dileptons that are not emitted as a pair from a parent hadron but emerge from semileptonic decays of two correlated daughter hadrons. Our calculations suggest a sizeable contribution of such sources in central heavy-ion collisions in the low-mass region. However, even the upper limits of our calculations are found to be far below the dilepton mass spectra of the PHENIX Collaboration.     

Mid-rapidity ϕ meson production at \sqrt {s_{NN} } = 200 GeV Au+Au and pp collisions from STAR

We present the results for the measurement of ? meson production in $\sqrt {s_{NN} } = 200$ GeV Au+Au and pp collisions at the Relativistic Heavy Ion Collider (RHIC). Using the event mixing technique, spectra and yields are obrained from the ?→K⁺K^? decay channel for five centrality bins in Au+Au collisions and in pp collisions. We observe that the spectrum shape in Au+Au collisions depends weakly on the centrality and the shape of the spectrum in pp collisions is significantly different from that in Au+Au collisions.     

Dilepton creation based on an analytic hydrodynamic solution

High-energy collisions of various nuclei, so called “Little Bangs” are observed in various experiments of heavy ion colliders. The time evolution of the strongly interacting quark-gluon plasma created in heavy ion collisions can be described by hydrodynamical models. After expansion and cooling, the hadrons are created in a freeze-out. Their distribution describes the final state of this medium. To investigate the time evolution one needs to analyze penetrating probes, such as direct photon or dilepton observables, as these particles are created throughout the evolution of the medium. In this paper we analyze an 1+3 dimensional analytic solution of relativistic hydrodynamics, and we calculate dilepton transverse momentum and invariant mass distributions. We investigate the dependence of dilepton production on time evolution parameters, such as emission duration and equation of state. Using parameters from earlier fits of this model to photon and hadron spectra, we compare our calculations to measurements as well. The most important feature of this work is that dilepton observables are calculated from an exact, analytic, 1+3D solution of relativistic hydrodynamics that is also compatible with hadronic and direct photon observables.     

Evidence for a Soft Nuclear Equation-of-State from Kaon Production in Heavy-Ion Collisions

The production of pions and kaons has been measured in 197Au+197Au collisions at beam energies from 0.6 to 1.5A GeV with the kaon spectrometer at SIS/GSI. The K+ meson multiplicity per nucleon is enhanced in Au+Au collisions by factors up to 6 relative to C+C reactions, whereas the corresponding pion ratio is reduced. The ratio of the K+ meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.     

Electromagnetic radiation and in-medium effects

The theory of thermal photon and dilepton emission from a hot and dense hadronic gas, as well as from the Quark-Gluon Plasma, is reviewed in the context of extracting in-medium properties of the matter constituents. In phenomenological applications to ultrarelativistic heavy-ion collisions we focus on recent photon and dilepton spectra as measured by WA98 and CERES/NA45, respectively, at CERN-SPS energies.     

Leptonic and charged kaon decay modes of the phi meson measured in heavy-ion collisions at the CERN super proton synchrotron

We report on results of a measurement of meson production in central Pb-Au collisions at E(lab) = 158A GeV. For the first time in the history of high energy heavy-ion collisions, phi mesons were reconstructed both in the K+K- and the dilepton decay channels in the same experiment. This measurement yields rapidity densities near midrapidity, from the two decay channels, of 2.05 +/- 0.14(stat) +/- 0.25(syst) and 2.04 +/- 0.49(stat) +/- 0.32(syst), respectively. The shape of the measured transverse momentum spectrum is also in close agreement in both decay channels. The data rule out a possible enhancement of the phi yield in the leptonic over the hadronic decay channel of a factor 1.6 or larger at the 95% C.L. This rules out the discrepancy reported in the literature between measurements of the hadronic and dimuon decay channels by two different experiments.     

Measurement of electron-muon correlations from semi-leptonic D decay in 200 GeV p + p collisions at RHIC-PHENIX

Charm production is a valuable probe of the early stages of a heavy ion collision. Correlated electron-muon pairs are a signature of semi-leptonic D decays, and a measurement of D mesons provides information on charm quark energy loss in the hot medium. The energy loss of heavy quarks is still not fully understood, so it is vital to investigate different decay channels of charm mesons to better understand this process. Measurements of electron-muon pairs suffer less from background than e ⁺ e ⁻ or μ⁺μ⁻ pairs since neither direct lepton production nor resonance decays produce this type of correlated signal. Another advantage is that because electrons are measured in the central arms and muons are measured in the forward region in PHENIX, open charm can be probed in a rapidity region different from previous dilepton measurements. Studying electron-muon pairs in p + p collisions provides an important baseline for the study of these processes in d + Au and Au + Au collisions. The data in this analysis was obtained during the 2006 RHIC run of p + p collisions at 200 GeV. The current status of this analysis will be presented.     

Charged particle production in proton-, deuteron-, oxygen- and sulphur-nucleus collisions at 200 GeV per nucleon

The transverse momentum and rapidity distributions of net protons and negatively charged hadrons have been measured for minimum bias proton–nucleus and deuteron–gold interactions, as well as central oxygen–gold and sulphur–nucleus collisions at 200 GeV per nucleon. The rapidity density of net protons at midrapidity in central nucleus–nucleus collisions increases both with target mass for sulphur projectiles and with the projectile mass for a gold target. The shape of the rapidity distributions of net protons forward of midrapidity for d+Au and central S+Au collisions is similar. The average rapidity loss is larger than 2 units of rapidity for reactions with the gold target. The transverse momentum spectra of net protons for all reactions can be described by a thermal distribution with ‘temperatures’ between MeV (p+S interactions) and MeV (central S+Au collisions). The multiplicity of negatively charged hadrons increases with the mass of the colliding system. The shape of the transverse momentum spectra of negatively charged hadrons changes from minimum bias p+p and p+S interactions to p+Au and central nucleus-nucleus collisions. The mean transverse momentum is almost constant in the vicinity of midrapidity and shows little variation with the target and projectile masses. The average number of produced negatively charged hadrons per participant baryon increases slightly from p+p, p+A to central S+S,Ag collisions. Received: 28 October 1997 / Published online: 10 March 1998